The hippopotamus is a semi-aquatic mammal that spends a significant portion of its life submerged in the rivers and lakes of sub-Saharan Africa. While often mistaken for a strong swimmer, the adult hippo is actually built for a different kind of aquatic existence. Hippos generally do not float; their bodies are engineered to be negatively buoyant, meaning they are designed to sink rather than float freely on the water’s surface. This unique trait is a consequence of specific biological adaptations that allow the massive animal to interact with its watery habitat.
The Density Secret: Why Hippos Don’t Float
The primary reason adult hippos do not float like many other marine mammals lies in their extreme body density. Hippos possess massive, dense skeletons that function as a natural ballast, counteracting the buoyancy that their sheer size would otherwise create. This skeletal specialization is known as pachyostosis, where porous bone is replaced with more compact, heavier bone material.
This dense bone structure, combined with a high muscle mass and relatively low subcutaneous fat, results in a specific gravity greater than water. Unlike whales or seals that rely on thick blubber for insulation and buoyancy, hippos have a body composition that favors sinking, a characteristic that is advantageous for their lifestyle. This adaptation allows them to achieve a state of near-neutral buoyancy, letting them remain submerged with minimal effort. They can even regulate their sinking speed further by controlling the air in their lungs, creating a more negative buoyancy by exhaling before submerging.
Underwater Locomotion: The Riverbed Walk
Since they are not built to float or swim in the conventional sense, hippos have developed a unique method of moving through the water known as bottom-walking. Instead of using propulsive strokes to move across the water’s surface, they use their short, powerful legs to walk, trot, or even gallop along the substrate of the riverbed. This method of movement is facilitated by the water, which reduces the effective weight of the hippo, making their movement analogous to locomotion in a microgravity environment.
When moving at a faster pace, a hippo may employ a bounding or “slow-motion gallop” gait, where only one foot is in contact with the ground at a time, allowing for extended unsupported intervals. This bounding action, often seen in deeper water, can look like swimming from a distance as the animal pushes off the bottom, briefly becoming airborne before sinking again. Their average horizontal speed underwater during this bottom-walking has been measured at around 0.47 meters per second.
Specialized Adaptations for Submerged Life
The hippo’s body is finely tuned for spending the heat of the day submerged, often with just the top of its head exposed to the air. A key adaptation is the strategic placement of their sensory organs: their eyes, ears, and nostrils are positioned high on the roof of the skull. This allows the animal to see, hear, and breathe while the rest of its massive body remains protected underwater.
When fully submerged, the hippo can seal its nostrils and ears with muscular sphincters to prevent water from entering. Their breath-holding capacity is substantial, with adults able to remain underwater for an average of four to six minutes, and sometimes up to eight or ten minutes. A remarkable reflex allows a submerged hippo to sleep, automatically surfacing to take a breath and then sinking back down without actually waking up.
The hippo’s skin is highly sensitive and prone to rapid dehydration and sunburn when exposed to the African sun. To combat this, their skin secretes a thick, reddish-orange fluid containing pigments that act as a natural, moisturizing sunscreen and antibiotic. This need to remain cool and hydrated is the fundamental reason the hippo is so reliant on its aquatic habitat.